As known, according to standard procedure rules, an aircraft (for instance a civil transport airplane) switches from a descent start altitude to a final approach start altitude:
either while carrying out a descent at a constant speed, followed by a defined approach level, for instance, by an altitude of 3,000 feet (that is about 914 meters), for decelerating and then stabilizing at a predetermined intermediary speed, the aircraft maintaining this level, with this intermediary speed, until it intercepts a predefined final approach path Ai; or                while carrying out a continuous descent approach, wherein the deceleration level at a constant altitude is omitted, so that the aircraft descends and decelerates simultaneously, this step being optimally divided into several sections each having specific descent slopes.        
Intercepting the approach level, or the last segment of the approach in a continuous descent, and the approach path defines the initiation of the final approach phase.
The standard slope associated with this approach path and defined in relation to the ground (the reason why it will be referred to as “ground slope” later on) is usually set at −3°. During the approach phase, the aircraft decelerates again,keeping track of the approach path, while spreading the slats, the flaps and the landing gears, so as to exhibit a landing configuration. At approximately 1000 feet (that is about 305 meters), the aircraft keeps a stabilized approach at a predefined approach speed (being, more specifically, a function of the configuration of the aircraft and of the meteorological conditions) up to 50 feet (that is about 15 meters), and then initiates its flare so as to join the runway and complete the landing.
It is known as well that, in order to avoid obstacles (for instance formed by the relief, buildings, etc.), an increased ground slope approach phase (that is for instance, switching from a −3° standard ground slope to a −4° ground slope) could be carried out. It should be noticed that, whatever the final ground slope to track, the latter is published in the operational approach procedures as defined by the air authorities.
It is also known that, in addition to air safety considerations, an increased ground slope approach phase enables to reduce the environmental impacts in the vicinity of airports (including noise and polluting emissions), as, thru the geometric structure, the aircraft flies higher for a same distance to the threshold of the runway and that the motor speed necessary to maintain this slope is lower in general. This explains why the different actors of the aeronautic field (aircraft manufacturers, airports, air companies) are eager to develop increased ground slope approaches.
Furthermore, it is known that transport civil dedicated aircrafts generally carry out their final approach on a ground slope set at −3°, while being certified for flying up to −4.49° ground slopes. Beyond this slope value, the approach phase is considered, by the international rules, as an approach on a steep slope and the aircraft should suit additional certification requirements.
Although such increased ground slopes (that is higher than −3° but lower than −4.5°) are regularly followed on numerous international airports, in order to avoid obstacles, it is not usual for the aircraft to land abruptly (this is referred to, in such a case, as a “hard” landing), comprising the good behaviour of the aircraft, especially when such hard landings are daily occurring.
In other words, in order to stand up to regular increased ground slope approaches (equal for instance to −4°), it is indispensable to review the design criteria of the aircraft in terms of performance, maneuverability, or even of structure, so as to ensure a secured landing, whatever the characteristics of the aircraft, the meteorological conditions and the geographical situations in the vicinity of airports.
Indeed, increasing the ground slope during a final approach results, on the one hand, in an increase of the vertical speed of the aircraft in relation to the ground (also referred to as “ground vertical speed” subsequently) and, on the other hand, in a decrease of the deceleration abilities of the aircraft (at the origin of hard landings). It can, for instance, be shown that, in the case of a conventional speed Vgs, a −1° increase of a ground slope initially at −3° (that is an increased ground slope equal to) −4° could result in the vertical speed Vz increasing By more than 30%.
An increase of the ground slope (and thus of the vertical ground speed) involves a review of maneuverability and deceleration abilities, even redimensioning landing gears, resulting in an additional embedded load, important modifications of the systems of the aircraft, as well as the need of an adapted training of pilots.
The present invention aims at solving these drawbacks.
To this end, according to this invention, the method for optimizing the landing of an aircraft on a runway, said landing comprising an approach phase, defined by an approach path to be tracked with which a predefined ground slope is associated, and a flaring phase, is remarkable in that:                in a preliminary step:        a target vertical speed in relation to the ground to be applied to said aircraft upon the initiation of the flaring phase is defined on the basis of performances and characteristics specific to said aircraft; and        as a function of said target vertical speed and of at least one outside parameter, an optimized ground slope, associated with the approach path, is determined which is higher than or equal to the predetermined ground slope, and        as soon as the approach path is intercepted by the aircraft, said aircraft is guided so as to track the determined optimized ground slope, associated with said approach path, and to reach the previously defined target vertical speed at the initiation of the flaring phase.        
Thus, thanks to this invention, the ground slope of the approach path is optimized, during the approach phase, while determining an optimized ground slope (with respect to the ground slope issued from standard procedure rules) from a target vertical speed predefined, based on characteristics being specific to the aircraft and one or more outside parameters, such as those associated with meteorological conditions, environmental conditions and characteristics specific to the aircraft.
Indeed, it has been shown that the flare carried out upon a landing of an aircraft depends nearly exclusively on the ground vertical speed of the aircraft, so that is forms an efficient parameter for characterizing the flare and provides an indication on the ability of the aircraft to ensure a secured landing and to avoid an inappropriately throttling up. The present invention is advantageously based on the fact that the above mentioned outside parameters disturb the deceleration abilities of the aircraft, at a set ground slope, and increase the risk that the aircraft should abruptly land on the runway, so that taking the latter into consideration in the calculation of the optimized ground slope enables to reduce the risk of hard landings.
In other words, setting the ground vertical speed of the aircraft upon the initiation of the flare (at about 50 feet) to a preliminarily defined nominal target value, the present invention will secure the final approach phase, providing a more constant, repeated and easier flare, while increasing the slope, making advantageously use of the conditions of the approach being considered for improving the environmental aspects, without imposing new designing constraints.
The higher the ground slope of the approach, the lower the motor speed of the aircraft along the approach path, reducing the atmospheric and sound pollution, as well as the fuel consumption of the aircraft.
In addition, the optimizing method of the present invention also has the advantage of being able to be implemented:                readily in any aircraft;        without any structural modification of the aircraft;        without modification of the piloting laws or of the aerodynamic configuration of the aircraft;        without modification of operational procedures;        without impact on the air traffic control;        without modification of the airport facilities on the ground; and        without additional certification specific to this concept.        
Preferably, the outside parameter(s) belong to the group of parameters comprising:                the calibrated airspeed CAS of the aircraft with respect to the air. This speed CAS is a function of the bulk of the aircraft and of the flight configuration of the aircraft associated with the approach phase, so that, involving the speed CAS in the determination of the optimized slope, these last two parameters (bulk M, flight configuration) are taken into consideration;        the outside temperature at a standard height;        the horizontal speed of the wind;        the inclination of the runway with respect to the horizontal; and        the altitude of the runway.        Preferably, the optimized ground slope is determined from the target vertical speed, the calibrated airspeed CAS, the horizontal speed of the wind, the outside temperature at a standard height, as well as from the inclination and the altitude of the runway.        
In addition, the horizontal speed of the wind, taken into consideration during the determination of the optimized ground slope, belongs to a determined range of values able to be obtained from several technological solutions.
Furthermore, for determining the optimized ground slope preferably the following steps are carried out:                the density of the air at the standard height is determined from the outside temperature and from the altitude of the runway;        the true speed TAS of the aircraft with respect to the air is determined from the speed CAS and from the determined density of the air;        the optimized ground slope is determined from the target vertical speed, from the determined true speed TAS, from the horizontal speed of the wind and from the inclination of the runway.        
In a particular embodiment, the determination of the optimized ground slope is obtained thru geometric construction of a speed triangle.
Moreover, the target vertical speed could be defined preliminarily for each type of aircraft.
So as not to decrease the safety margins imposed by the air safety authorities, the optimized ground slope ranges between a predefined lower extreme value and a predefined higher extreme value, preferably equal respectively to −3° and to −4.49°.
Furthermore, the horizontal speed of the wind could be obtained according to at least one of the following ways:                thru measurement of the wind at the level of the control tower of the runway being considered, without taking gusts into consideration;        thru retrieving data measured directly by one or more other aircrafts located in the vicinity of the runway.        
The present invention further relates to a device for optimizing the landing of an aircraft on a runway, said landing comprising an approach phase, defined by an approach path to be tracked with which a predefined ground slope is associated, and a flaring phase. According to this invention, such a device comprises:                means for determining, as a function of at least one outside parameter and of a target vertical speed, preliminarily defined from performances and characteristics being specific to said aircraft, an optimized ground slope associated with the approach path to be tracked being higher than or equal to the predefined ground slope; and        means for guiding the aircraft as soon as the latter intercepts the approach path, so that it can track the determined optimized ground slope associated with said approach path, and it reaches the preliminarily defined target vertical speed during the initiation of the flaring phase.        
Moreover, as the optimized ground slope is determined from said target vertical speed, the calibrated airspeed CAS, the horizontal speed of the wind, the outside temperature at a standard height, as well as the inclination and the altitude of the runway, said determination means preferably comprise:                means for calculating the density of the air at the standard height as a function of the outside temperature and of the altitude of the runway;        means for calculating the true speed TAS of the aircraft with respect to the air from the speed CAS and from the determined density of the air; and        means for calculating the optimized ground slope from the target vertical speed, from the determined true speed TAS, from the horizontal speed of the wind and from the inclination of the runway.        